This proposal aims to determine whether outbreaks of eastern equine encephalitis virus (EEEV) develop mainly among birds that attain high local abundance, are faithful to particular roost sites, are probed repeatedly by vector mosquitoes, are virus-competent and often die as infection wanes; these properties point towards starlings and perhaps grackles or robins. Reservoir relationships of EEEV will be compared with those of the co-transmitted and more rapidly amplifying Highlands J virus (HJV). The study will: (1) determine where numerous birds roost communally near southeastern Massachusetts swamps, in which enzootic vector mosquitoes (Culiseta melanura) breed, and describe distance and direction of avian daily dispersal; (2) define the reservoir characteristics of starlings, grackles and robins for these arboviruses by comparing virus-competence, frequency of feeding contact by enzootic vector mosquitoes with these birds, and frequency of interruption of such contact. The duration of vector contact required for virus transmission also will be measured; (3) identify the venue of virus transmission in nature by comparing the density of vector mosquitoes in roost sites to that in other sites and by determining whether these arboviruses most frequently infect enzootic vector mosquitoes in the vicinity of massive communal roost sites; (4) relate the density of larval enzootic vector mosquitoes with the maturity of the swamps in which they breed, to examine the notion that the emergence of EEE was promoted by the 20th century return of cedar swamps, which paralleled the Nearctic proliferation of starlings. The proposed effort tests the concept that EEEV may perpetuate mainly where numerous virus-competent birds roost on successive nights, especially if vector-host contact is frequent and such birds tend to die as their infectivity wanes. The research seeks a general definition of reservoir capacity and anticipates contributing to improved suppression of EEEV outbreaks.